In the manufacture of large format press plates as used in map making, it has been conventional to produce these plates by direct contact printing with large format transparencies and a very high intense arc light source. The use of large format transparencies has been required because projection printing with arc light sources would require power levels which would burn up a small scale transparency. Further, illumination of an entire transparency with an enlarged laser beam for projection printing is also impractical because of the very high laser power levels that would be required. Further, in any contemplated whole transparency laser system, there would be laser speckel artifacts which would derrogate the fidelity of the plate.
Also known in the art are laser projection systems which rely upon various techniques to modulate the laser beam as it is scanned across the plate. Such a system is shown in U.S. Pat. No. 3,836,709--Hutchison. In modulation systems such as that shown in the Hutchison patent, there is first required use of a partially reflecting mirror (beam splitter) which must be placed between the imaging lens and the recording medium, or press plate. This, however, is a poor optical practice which leads to geometric distortions of the image projected onto the recording medium because of refractions in the mirror plate due to off axis light beams. Further, if such a plate were not flat to within a portion of one wavelength, there will be phase effects which result in additional geometric distortions of the projected image. Further, in systems such as the one shown in Hutchison, the input transparency must be translated in two orthoganal directions by a transparency driving mechanism 18 for the purpose of generating precise control signals for the scanning apparatus. Any noise or nonlinearities or drift will result in geometric distortions of the projected image.
Systems such as Hutchison are complex, which results in higher costs, and have lower levels of projection fidelity.
In the art of manufacturing mapping press plates, systems such as the one shown in the Hutchison patent are not useful because of their inherent inaccuracies. In such map plates, there is a requirement for extreme accuracy of the reproduction, and elimination of any geometric distortions.
The present contact printing production techniques used in the manufacture of maps may require as many as twenty 24".times.30" photographic film sheets to prepare color separated negatives associated with one topographic map. The sheets are relatively expensive, and the voluminous storage requirements are substantial.